Methane comprising fluids can be derived from a number of sources, such as natural gas or petroleum reservoirs, aerobic or anaerobic digestion of biological material, or from a synthetic source such as a Fischer-Tropsch process.
Aerobic or anaerobic digestion of biological material can have many forms. It may, for example, concern aerobic or anaerobic digestion of dairy waste. Aerobic or anaerobic digestion of biological material results in a methane comprising fluid which may also be referred to as bio-methane, bio-gas or bio-fuel.
The Fischer-Tropsch process can be used for the conversion of synthesis gas (from hydrocarbonaceous feed stocks) into liquid and/or solid hydrocarbons. Generally, the feed stock (e.g. natural gas, associated gas and/or coal-bed methane, heavy and/or residual oil fractions, coal, biomass) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas).
In this text the term natural gas is used to refer to methane comprising streams originating from any source.
The term methane comprising fluid is used in this text to refer to fluids comprising at least 40 m/m % methane. The term in particular relates to natural gas fluids. Similarly, the terms methane comprising gas, methane comprising liquid, and methane comprising slush of liquid and solids, refer to gas, liquid, and slush of liquid and solids comprising at least 40 m/m % methane. A methane comprising fluid may comprise up to 100 m/m % methane, especially up to 99.9 m/m % methane, it may comprise up to 99 m/m % methane, it may comprise up to 90 m/m % methane. In the context of this invention a slush of liquid and solids is considered a fluid. The term methane comprising gas in particular relates to natural gas. The terms methane comprising liquid, and methane comprising slush of liquid and solids, in particular relate to LNG.
Natural gas is a useful fuel source, as well as a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream to enable compact storage of the natural gas and/or efficient transport of the natural gas over long distances. Natural gas can be more easily stored and transported in a liquid form than in a gaseous form because it occupies a smaller volume.
Liquefied natural gas plants are well known in the field and comprise the following processing steps:                optionally treating the methane comprising stream by removing impurities in a treating stage, such as water, acid gases, mercury,        optionally removing natural gas liquids from the methane comprising stream in a NGL stage, such as ethane, propane, butane and heavier components,        cooling the methane comprising stream in one or more cooling stages, for instance a pre-cooling stage and a main cooling stage, and        optionally flashing the methane comprising stream in an end-flash stage and,        optionally, storing the liquefied natural gas in a storage tank.        
Methane comprising gas, such as natural gas is obtained from various sources. After liquefaction it is transported worldwide as LNG. LNG and regasified LNG are used for different purposes.
Different countries and different purposes may have different requirements. This may relate to safety, proper use and/or tax regimes.
There is a need for a possibility to trace methane comprising liquid and slush. There thus is a need for a method of labeling, or marking, methane comprising liquid and slush.